1. Field of the Invention
The present invention relates to a liquid dispensing apparatus. More particularly, the present invention relates to a liquid dispensing apparatus in which a surface level of a liquid to be dispensed requires to be measured when a dispending probe is descended, an automatic analyzer using the liquid dispensing apparatus, and a liquid surface detecting apparatus.
2. Description of the Related Art
In a liquid dispensing apparatus for sipping a predetermined amount of a liquid in one container and discharging the sipped liquid into another container by using a probe, it is important that, when sipping the liquid, the probe be stopped at a position slightly below a surface level of the liquid to be sipped. If the probe is descended through a stroke to reach a deeper position from the liquid surface level, an increasing risk arises in that the accuracy of the dispensed liquid deteriorate because a larger amount of the liquid attaches to an outer periphery of the probe and the attached liquid drops at the same time as when the sipped liquid is injected into another container. Another possible risk is the so-called carryover that different kinds of liquids mix with each other via the probe.
A technique for precisely detecting the liquid surface is therefore important. In one known liquid dispensing apparatus, electrostatic capacitance between a dispensing probe and a liquid is measured, and a liquid surface is detected by confirming a change of the electrostatic capacitance caused when the dispensing probe contacts the liquid surface. With the known device, however, liquid surface information is obtained only after the dispensing probe has reached a position very close to the liquid surface or has come into contact with the liquid surface. In other words, because the probe enters the liquid without being sufficiently decelerated, there is a possibility that the interface between the probe and the liquid surface may be unstable at the time of sipping the liquid, and hence the dispensing accuracy may deteriorate. If the probe descent speed is slowed down to avoid the above possibility, this causes another problem of a reduction in the dispensing speed.
To overcome the above-described problems, several methods for detecting a liquid surface level in a non-contact manner are proposed. For example, Patent Reference 1; JP,A 10-2904 discloses a method comprising the steps of installing a light source and a photosensor, arranging optical fibers so as to extend from them up to a probe tip, and receiving a light reflected from a liquid surface by the photosensor, thereby detecting a position of the probe tip relative to the liquid surface. Also, Patent Reference 2; JP,A 11-83867 discloses a method comprising the steps of irradiating an ultrasonic wave toward a liquid surface and computing a liquid surface level based on a time that has lapsed until the ultrasonic wave reflected from the liquid surface is received.